The IEEE (Institute of Electrical and Electronic Engineers) 802.11 standards provide guidelines for allowing users to wirelessly connect to a network and access basic services provided therein. As well, IEEE 802.11 standards provide guidelines for multicast transmissions sent via the wireless network.
The IEEE 802 standards also provide protocol directed toward the use of virtual local area networks or virtual LAN's (VLANs) in wireless networks. Virtual networking refers to the ability of switches and routers to configure logical topologies on top of the physical network infrastructure allowing any arbitrary collection of LAN segments within a network to be combined into an autonomous user group, appearing as a single local area network (LAN).
VLANs offer significant benefits in terms of efficient use of bandwidth, flexibility, performance, and security. VLAN technology functions by logically segmenting the network into different “broadcast domains” whereby packets are only switched between ports that are designated for the same VLAN. Thus, by containing traffic originating on a particular LAN only to other LAN's within the same VLAN, switched virtual networks avoid wasting bandwidth. Conventionally, this is a drawback inherent in traditional bridged/switched networks where packets are often forwarded to LAN's that do not require them.
The VLAN approach also improves scalability, particularly in LAN environments that support broadcast- or multicast-intensive protocols as well as other applications that flood packets throughout the network.
The Internet Engineering Task Force (IETF) has published an Internet Group Management Protocol (IGMP) standard, which defines a method for organizing IP nodes into an IP multicast group. An IP multicast group is identified by an IP multicast address. An IP node joins an IP multicast group by transmitting an IGMP Membership Report on its local subnet. When an IP Multicast Router receives an IP multicast packet, it only forwards the packet onto other subnets where there are members of the IP multicast group identified by the destination IP multicast address.
Conventionally, the 802.11 standard for wireless networks presumes support for a single group key (e.g. VLAN) for a client. An 802.11i-compliant AP may be configured to send a Group Key to an 802.11i station. This Group Key is conventionally sent in an EAPOL Key message in accordance with the IEEE standards.
Additionally, the EAPOL Key message may contain an integer Key ID, which identifies the Group Key. An 802.11i transmitter enters the Key ID of the key used to encrypt a transmitted 802.11 multicast frame into a Key ID field in the 802.11 frame header. The 802.11 receiver uses the Key ID to select the correct key to decrypt the multicast frame.
In accordance with traditional methods, “Layer 2 Broadcast Domain” architecture may be configured to correspond to a single Internet Protocol (IP) subnet or VLAN. An IP Multicast Domain may be configured to span multiple subnets. Therefore, Ethernet and 802.11 stations on multiple VLANs may be members of the same multicast group.
An 802.11 access point (AP) may be connected to an Ethernet LAN on a VLAN trunk link whereby each VLAN enabled on an AP Ethernet link may correspond to an 802.11 broadcast domain. In traditional systems, an AP is configured to use a different set of 802.11 broadcast encryption keys for each 802.11 broadcast domain. These broadcast domain specific encryption keys prohibit 802.11 stations in a first broadcast domain from receiving broadcast frames transmitted on a second broadcast domain.
Currently, there is not a distinction between such a VLAN-based broadcast domain and an IP Multicast Domain. Therefore, an AP will often receive multiple copies of the same IP multicast packet on its Ethernet link (e.g. one copy for each VLAN where the respective multicast group is active). Accordingly, an AP will often transmit multiple copies of the same IP multicast packet to associated 802.11 stations.
Redundant multicast transmissions are problematic on 802.11 links. Useless multicast transmissions may excessively consume bandwidth. If simple rate-limiting (e.g. as in the current AP350 implementation) is used to control the amount of bandwidth used for multicast transmissions, both useful and useless multicast frames may be discarded.
An additional problem associated with traditional methods is that if there is a single power-save station associated to an AP, all multicast frames are buffered and transmitted immediately following an 802.11 beacon. Accordingly, higher-priority Quality-of-Service (QoS) unicast transmissions may be delayed for the duration of the multicast delivery period. Power-save stations must stay awake, for the duration of the multicast delivery period, to receive multicast transmissions; therefore, multicast transmissions can reduce battery life in power-save stations.